A capacitor type welding device is advantageous in that the capacitor type welding device does not need a large facility for receiving electric power, if compared to a general AC welding device, because the capacitor type welding device stores welding electric power in a welding capacitor in a longer time than a discharge time and discharges the electricity at once. The capacitor type welding device is also advantageous in that welding marks (burning) are hardly created at welded portions and distortions are small because the object(s) to be welded is (are) only superheated to a small extent. Because of such advantages, the capacitor type welding device is employed for any size of industrial facility from small to large industrial facilities.
In general, the capacitor type welding device uses a capacitor bank as a welding capacitor, which includes a plurality of parallel-connected electrolytic capacitors. Because the welding method using the capacitor type welding device is well known, the welding method is not described here in detail. In brief, an object (objects) to be welded is (are) placed between welding electrodes, the distance between the welding electrodes is made smaller, and a predetermined welding pressure is exerted on the object(s) by the welding electrodes. The welding capacitor is charged while such mechanical operation is being performed.
When the charged voltage of the welding capacitor rises to a predetermined value, the charging circuit is turned off to interrupt (discontinue) the charging process. While the welding electrodes are applying the welding pressure on the object(s) to be welded, a discharge switch is turned on. As a result, a steeply increasing pulse current flows in the primary winding of the welding transformer. The turn of the secondary winding of the welding transformer may be one or the like, which is significantly smaller than the turn of the primary winding. Thus, a pulse welding current that is significantly larger than the current of the primary winding flows in the secondary winding and the object(s) to be welded, so that the welding is performed with such welding current and a welding product is obtained in a short time.
In general, the capacitor type welding device uses a welding transformer, and connects the welding capacitor in series to the primary winding of the welding transformer in order to avoid the bias excitation of the welding transformer. Both the charging current to the welding capacitor and the discharge current in the opposite direction flow in the primary winding of the welding transformer. Apart from the bias excitation of the welding transformer, a certain type of capacitor type welding device connects an inductor between the charting circuit and the welding capacitor to improve a power factor such that the charging current does not flow through the primary winding of the welding transformer but the charging current flows in the welding capacitor through the inductor from the charging current. Such configuration can charge the welding capacitor at a high efficiency in a stable manner. The welding transformer and the inductor may have the inductances that are suitable for an intended use. A route for the charging current to flow in the welding capacitor from the charging circuit (hereinafter, referred to as “charging path”) also includes a floating (stray) inductance, which is present in the charging route, other than to the above-mentioned inductance.
The charging circuit may be a single-phase or a three-phase hybrid bridge full wave rectifier circuit, which includes thyristors and rectifier diodes connected in a bridge structure, if a certain charging method is employed for charging the welding capacitor or for other reasons. The charging current is controlled by controlling a conduction angle of the thyristor(s) (see, for example, Patent Literature 1). When the thyristors are used in the charging circuit and the welding transformer or the above-mentioned inductor(s) is (are) provided on the charging path, a magnetic energy is stored in their inductances and/or the floating inductance (hereinafter, referred to as “inductance of the charging path”). A current that is caused to flow by this magnetic energy (hereinafter, referred to as “backflow current” or “return current”) adversely affects the charging circuit as will be described below.
In a common welding process, a large welding current is often needed. This is also true to the capacitor type welding device. In case of a highly efficient capacitor type welding device that includes the hybrid bridge full wave rectifier circuit as the charging circuit, the respective thyristors of the hybrid bridge full wave rectifier circuit are phase controlled such that the switching of the thyristors is carried out at certain cycles (e.g., more than ten cycles and less than several tens of cycles, or even more) to charge the welding capacitor. For example, when constant current control is performed, a generally constant large charging current flows to the welding capacitor from the charging circuit through the primary winding of the welding transformer or the inductor every time the respective thyristors are brought into the conduction state, until the charged voltage of the welding capacitor reaches a predetermined value. At the same time, a magnetic energy is stored in the inductance of the charging path.
The backflow current which is caused to flow due to the magnetic energy flows in the same path as the charging path and therefore the backflow current flows through the hybrid bridge full wave rectifier circuit, through which the charging current flows. Particularly in case of a three-phase hybrid bridge full wave rectifier circuit, because a non-conduction time between the thyristors, which are brought into the conduction state in turn (successively), is short, the backflow current which flows in a certain thyristor due to the magnetic energy may not become smaller than a holding current even if the phase control signal drops to the zero level from a high level. In this case, this thyristor keeps the conduction state without recovering its forward-blocking function.
Particularly in case of the capacitor type welding device, the charging current flows in the welding transformer or the inductor in the predetermined direction for certain cycles (more than ten cycles and less than several tens of cycles, or even more) as described above, and therefore the magnetic energy stored in the inductance(s) of the welding transformer or the inductor may gradually increase. If this occurs, the backflow current due to the magnetic energy may become also large, a certain thyristor in the hybrid bridge full wave rectifier circuit may not be able to become a non-conduction state but keep the conduction state. Then, the desired control of the charging circuit becomes difficult.